Ridge - Either get one with grommets or tie downs as you can see in the last pic above. If you carry a poncho, you can tie it to one side as a wind block if the wind shifts on you, it gets cold or it's raining. Set your fire off center to one side then you can close the other side with your poncho and stay pretty warm.
If you have it made, you can even have flaps built so you can drop either side.
Here's another thread on tarps:
http://www.wilderness-survival.net/f...ht=canvas+tarp
Another:
http://www.wilderness-survival.net/f...ht=canvas+tarp